1. Field of Invention
This invention relates to vulcanizable rubber compounds and the cured products thereof characterized in containing a petroleum pitch as a plasticizing additive.
2. Prior Art
In modern rubber technology, plasticizers in the form of extender oils are widely used in formulating resilient type rubber compositions. These oils, when incorporated into a rubber composition, presumedly lubricate the molecules of rubber so that they slide easily on each other. Consequently, the rubber composition can be mixed and extruded through processing apparatus without rupturing of the individual rubber molecules and consequent degradation of desired rubber properties. U.S. Pat. No. 2,964,083, for example, discloses compositions of synthetic rubber containing preferably liquid or oily plasticizers, reinforced with carbon black and additionally containing the usual additives. The plasticizers disclosed generally are derived from the refining of petroleum and represent petroleum fractions obtained by distillation or extraction before these fractions have been subjected to any sort of decomposition by pyrolysis or other forms of cracking. The types of rubber in which plasticizers or extender oils are used include the styrene-butadiene, polybutadiene, polyisoprene, ethylene-propylene terpolymer and natural rubbers with the styrene-butadiene type predominating.
For practical and economic reasons rubber compounders have continued to increase the concentrations of petroleum extender oils in their rubber formulations, and the result has been improved processing characteristics of the rubber and enhanced mechanical performance of pneumatic tires and other fabricated rubber articles. There have been some indications, however, that the beneficial effects of oil extenders are not permanent. For example, increased wear, decreased adhesion and tread separation accompanying the prolonged service use of tires may result from the migration of diffusion of extender oil from a region of higher concentration to an adjacent region of lower concentration. The oil concentration gradients between different sections in a tire are designed to obtain the optimum balance of elastomer properties in the tire as a complete entity. Studies with laboratory-simulated tires containing radio-active-tagged extender oils have further corroborated the phenomenon of extender oil migration.